Long before the invention of portland cement, which led to the extensive utilization of concrete as a construction material, various reinforcement techniques were well known as a means of adding strength and stability to plaster, adobe and other such early cementious materials. These techniques were quickly adapted, modified and improved along with the growing use of concrete, which, while highly resistant to compressive force, lacks the tensile strength required for many construction uses. Reinforcement adds the necessary tensile strength.
The most common technique for reinforcing concrete involves the suspension of wire mesh or steel rods in the form or mold into which the liquid concrete is poured and cured.
Over the past half-century increasing use has been made of prestressed concrete, in which reinforcing tendons, generally of such high tensile strength material as hard drawn steel rods or cables, are stretched or tensioned within the form or mold either before the concrete is poured or after it is poured but still ductile. The tension of the prestressed tendons exerts a tensile force on the surrounding concrete imparting to it a tensile strength vastly superior to that of ordinary reinforcement rods. Among the advantages of prestressing, is the fact that less concrete is required in a prestressed beam or slab thus reducing its weight.
Prestressing, as currently practiced, is divisible into two general techniques; pretensioning and post-tensioning.
In pretensioning, the tendons are tensioned either before or immediately after the concrete is poured. One end of each tendon is anchored to one wall of the mold, extended across the mold and through the opposite wall. Either before the concrete is poured, or (more commonly) immediately after, the tendon is stretched or tensioned by a hydraulic jack or any other means of exerting a tensioning force on the unanchored end of the tendon which is extended through the wall of the mold. When optimum tension has been reached, the unanchored end of the tendon is anchored to the mold wall through which it extends. Since the liquid concrete offers little resistance to plastic deformation, the opposite walls of the mold must sustain the entire tensile force of the tendon stretched between them. When multiple tendons are employed, the tensile force is multiplied and the mold walls must be extremely rigid to resist bending or deforming. Such rigid forms or molds are expensive, cumbersome and require great care and skill in preparation and use. Because of this, pretensioning is generally practical only in factory casting, where the mold need not be moved and can be used again and again to form concrete structures of the same shape.
Pretensioning has one other characteristic disadvantage. A tendon can only extend in a straight line between its opposite anchored ends. It cannot generally be effectively employed in forming a curved slab or arcuate beam.
In post-tensioning, each tendon is positioned in the mold before the concrete is poured; but, unlike a pretensioned tendon, it is heavily coated with grease or some similar heavy lubricant which will prevent the concrete from adhering to the tendon. In most modern applications, the tendon is not only lubricated but surrounded by a plastic hose or sheath to assure that it will not become adhered to the concrete and will remain easily movable within the channel formed by the plastic sheath within the concrete even after the concrete has cured and hardened.
In contrast to pretensioning, the post-tensioned tendon remains inert, untensioned, while the concrete is very ductile.
It should be appreciated, at this point, that concrete is poured as a liquid and sets within twenty four hours into a relatively solid form, but the curing process takes much longer and even after a week the concrete is to some extent ductile. Even after many months and after being fully cured, concrete remains capable of some flow characteristics in response to forces exterted upon it.
In post-tensioning, the tendons are tensioned a week or so after the concrete has been poured, when the concrete is relatively solid and the form or mold has been removed. One end of the tendon is anchored to one end of the concrete structure through which it extends and the other end of the tendon which extends beyond the concrete structure is pulled by a hydraulic jack, or other means of exerting a tensioning force, until it has reached optimum tension and then the unanchored end of the tendon is anchored to the concrete structure at the point from which it extends.
Post-tensioning overcomes the two aforementioned characteristic disadvantages of pretensioning.
Because post-tensioned tendons are tensioned after the concrete is relatively solid, and the mold removed, a simple, inexpensive form or mold may be used, sufficient merely to contain the concrete while it is setting and curing and not necessarily so strong and rigid as to sustain the tensile force of pretensioned tendons. Such forms can be easily and inexpensively constructed on the site with less care and skill than required of a mold for pretensioned concrete.
Also because post-tensioned tendons are tensioned after the concrete is relatively solid, they do not necessarily have to extend in a straight line between their opposite anchored ends, but may be used to impart tensile strength to a curved or arcuate concrete structure. There are, however, limits as to the degree of curvature to which the application of post-tensioning is practical. In an extremely arcuate U-shaped beam, or hollow cylindrical shape like a culvert, the force exerted by tensioned tendons becomes counter-productive. The tensile force of such extremely curved tendons rather than imparting end-to-opposite-end tensile strength, work against the curvature of the structure tending to pull outwardly the legs of the U-shaped beam or collapse the walls of a culvert.
One of the principal problems in post-tensioning is the loss of tension due to "creep", which includes both creep of the tendon and creep of the anchor.
Creep of the tendon involves a relatively minor loss as the steel tendon gradually deforms in response to the tension.
Creep of the anchor involves a much more substantial loss. It results from both the anchor losing its grip on the tendon and from the loss of tension between the application of the anchoring device and its settling into the concrete structure. For instance, one anchoring device (referred to later herein and illustrated in FIGS. 4 and 5 of the Drawings) is a longitudinally divisible, two piece cylinder having gripping teeth or grooves on its inner periphery and being frusto-conically shaped on its outer periphery. When the tendon has been stretched to the optimum tension, the anchoring device is applied and held to that portion of the tendon that extends immediately beyond concrete structure, then as the tensioning force is released, the anchoring device is pulled by the tension of the tendon into the adjacent channel formed in the concrete structure. As it is pulled into the opening of the channel and due to its frusto-conical shape, with the smaller end of the device toward the channel, the device wedges into place driving the gripping teeth or grooves into the tendon and locking or anchoring the previously unanchored end of the tendon. During the process of applying the anchoring device, releasing the tension on the tendon, and allowing the anchoring device to settle into the opening of the adjacent channel, a significant loss of tension occurs.
Two factors control the amount of tension that can be applied to a tendon; the tensile strength of the tendon and the concrete's resistance to compressive force. Given concrete's relatively high resistance to compressive force and what is economically feasible for the material of which the tendon is formed, the controlling factor is generally the tensile strength of the tendon. While there are obviously variables in the two factors which define the optimum tension, as applied to the most commonly used hard drawn steel rods or cables, optimum tension is achieved at around 28,000 p.s.i. Once this optimum tension has been reached, the anchoring device applied and the tension released, the tension is diminished by the amount of loss due to creep which is principally the result of the anchoring process as descibed above.
It is important to appreciating the background of the present invention to understand that the anchor creep loss remains the same regardless of the length of the tendon, although the stretch of the tendon increases in direct proportion to is length. For instance, if a 100 foot tendon stretches 10 inches at 28,000 p.s.i. and loses 2 inches to anchor creep, there is only a 20% reduction in its tensile force. But if a 40 foot tendon stretches 4 inches and loses two inches to anchor creep, there is a 50% reduction in its tensile force. In a 20 foot tendon, the anchor creep loss equals the tension and the resulting concrete structure is merely reinforced and not post-tensioned. Therefore, post-tensioning has, in the past, been impractical for use in forming relatively small concrete forms. For a slab less than 20 feet across it is useless.